Plants with dense, branched root systems are generally stronger, healthier, and better able to survive transplantation than plants that do not have such dense, branched root systems. However, the roots of plants grown in containers tend to grow in a straight line, without branching, and the roots develop a characteristic container-circling structure as they fill the container.
Plants with container-circling root structures are not well suited to successfully survive transplantation because the roots generally continue to grow in a restricting line pattern once the circling growth pattern is established. As a result, the root system does not grow outward from the containerized root system to properly anchor the plant and the plant does not then seek nutrition and moisture from a large enough area to support itself as it grows. Furthermore, plants having this type of circling root structure are not readily transported because the portion of the root that is in contact with the container is exposed to the high temperatures that such containers, which are generally formed of a black plastic, will reach in the sun. These high temperatures are damaging to the roots, which have limited recuperative powers due to the lack of branching.
It is known that certain copper chemicals will terminate a root's growth on contact, thereby causing the root to branch in a manner similar to that in which the trimming of a tree causes a single branch to become many branches. This is desirable in the plant nursery trade as a plant grown in a container having these type of copper chemicals will have a dense, branched root ball or system. Such a dense, branched root ball will result in a stronger, healthier plant that is better able to survive transplantation.
It is conventional practice that to promote root branching, the interior surfaces of plant containers used at nurseries are coated with paints, thermoset coatings, or other fluid materials containing copper. The coating is often applied at the nursery. However, this practice is problematic because of the exposure of nursery workers to paint sprays and the like. Government regulations restrict such exposure and these restrictions, along with the time and resources required for application, represent an excessive burden to nurseries. Prior to this invention, however, no commercially viable alternative to container surface coating has been available to nurseries.
Copper is toxic to plants, so copper available to promote plant root branching should not dissolve and leach into plant soil. Copper in the form of a water-insoluble copper salt is suitable to reduce or essentially eliminate copper toxicity. In order to be available for root growth control, the water-insoluble copper salt may be fixed in the container such that it will not significantly slough from the container, yet be sufficiently exposed such that plants roots recognize the copper. Prior art methods to fix copper in a container utilize thermoset coatings containing insoluble copper salts. These thermoset coatings generally must have good water resistance to prevent cracking and sloughing of the coating.
Incorporation of copper or other metal biocides into a thermoplastic composition is known. Growth of marine flora and fauna on submarine structures such as boats hulls and piers is inhibited using copper metal at high concentrations, wherein the copper is the major component of a thermoset or thermoplastic composition. Inclusion of copper or other metal salts in relatively low or trace concentrations in a thermoplastic imparts improved weatherability and heat resistance, and promotes environmental degradation in some thermoplastics. However, the use of copper as a copper insoluble salt incorporated into a thermoplastic composition as an effective plant root branching promoter in plant containers has not been taught.
It is desirable then to provide a thermoplastic composition comprising copper such that plant containers made using the composition will be effective in promoting plant root branching.